1. Field of the Invention
This invention relates to magnetic bubble domain structures, in general, and to an improved switch structure, in particular.
2. Prior Art
Several state of the art bubble replicators operate in what is called the cut and transfer mode. Among these replicators are the pickax and the sideways switches. In these switches the bubble stretches along its propagation path under the influence of the strong pole distribution on an oversized permalloy element that constitutes part of the switch. The replication is effected by applying to the control conductor a composite current pulse consisting of a large narrow pulse followed by a low current plateau that lasts for a little over a quarter of a field cycle. The bubble is elongated normal to the control conductor and is cut into two parts (i.e. two bubbles). The trailing bubble is subsequently transferred with the aid of the low current plateau into the secondary propagation track which is placed near the switch.
Considerable progress has recently taken place in the design of field access bubble domain devices with the introduction of the gap tolerant structure based on the half disk pattern and its variations. See, for example, I. S. Gergis, P. K. George, and T. Kobayashi, "Gap Tolerant Bubble Propagation Circuit", IEEE Trans. Magnetism MAG-12, p. 651 (1976). The fact that the minimum feature of this pattern is about 1/8 of the circuit period makes it possible to fabricate 8 .mu.m period devices using conventional photolithographic techniques. P. I. Bonyhard and J. L. Smith, "68 K Bit Capacity 16 .mu.m Period Magnetic Bubble Memory Chip Design with 2 .mu.m Minimum Features", IEEE Trans. Magnetism MAG-12, p. 614 (1976) reports a 16 .mu.m period switch design, based on the pickax element, which operates with adequate bias and phase margins. However, this design suffers the disadvantages of upper and lower margin degradation in the minor loop propagation due to failures at the pickax element as discussed in their publication. Also, non-volatile start/stop operation requires a relatively large holding field to alleviate the degradation in the operation due to the failure caused by the bubble attraction to the pickax tail, or one pole compensating bars, during the drive-field shut down.
In scaling down the pickax switch design to an 8 .mu.m period, several additional problems arise. The drive field required for successful operation increases to more than 50 Oe, mainly due to the increase in bubble-bubble interaction in the unevenly spaced tracks of the minor loops, as a result of the increase in the garnet magnetization. The control conductor width is reduced to less than 2 .mu.m making it difficult to reproduce, especially with the requirement of the conductor edge sloping for proper permalloy step coverage. It is highly desirable to eliminate or avoid these problems.